The widespread use of the Internet has led to a rapid increase in traffic volume for backbone communication systems, creating a desire for realization of practical optical communication systems operating at ultra-high speed exceeding 100 Gbps. One technology attracting attention to realize ultrafast optical communication systems is the digital coherent scheme that combines an optical phase modulation scheme with a polarization multiplexing and demultiplexing technique.
PTL 1 and NPL 1 respectively disclose techniques to compensate for a frequency deviation in digital coherent receivers. The invention described in NPL 1 allows for compensation for a frequency deviation by using local oscillation light whose oscillating frequency can be controlled, so as to control the oscillating frequency of local oscillation light in the opposite direction to a frequency deviation estimated value; however, the invention requires a configuration for controlling the oscillating frequency of local oscillation light.
PTL 1 discloses compensation for waveform distortion by performing overlap-type fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT).
The digital coherent receiver described in PTL 1 has circuitry which includes an input unit, an FFT input frame generation unit, an FFT processing unit, a characteristic multiplication unit, an IFFT processing unit, an IFFT output frame extraction unit, and an output unit.
It is assumed here that input data consists of 256 parallel signals and that the window size for FFT and IFFT is 1,024. The input data (time domain: 256 samples) is inputted to the input unit. The input unit buffers the incoming input data and generates a frame consisting of 512 samples every two clocks.
The input unit outputs the generated frame to the FFT input frame generation unit.
The FFT input frame generation unit generates, with respect to sample frames outputted from the input unit, a frame consisting of 1,024 samples by combining the current 512-sample frame with the immediately preceding 512-sample frame. The FFT input frame generation unit outputs the generated frame to the FFT processing unit.
The FFT processing unit transforms the frame outputted from the FFT input frame generation unit into frequency-domain data. The FFT processing unit outputs the transformed frame to the characteristic multiplication unit. The characteristic multiplication unit multiplies characteristic parameters for each frequency component with respect to the frequencies corresponding to the frame outputted from the FFT processing unit (for 1,024 frequencies). The characteristic parameters are inputted, for example, from an external area. The characteristic multiplication unit outputs the multiplied frame to the IFFT processing unit.
The IFFT processing unit transforms the frame outputted from the characteristic multiplication unit into time-domain data. The IFFT processing unit outputs the transformed frame to the IFFT output frame extraction unit. Discontinuous points are included in the vicinity of the frame outputted from the IFFT processing unit.
Thus, the IFFT output frame extraction unit discards 256 samples each, i.e., a quarter of the window size, from the front and rear of a frame outputted from the IFFT processing unit. If discontinuous points are within the area discarded by the IFFT output frame extraction unit, no discontinuous points are generated in the output obtained by joining 512 samples that have not been discarded. The IFFT output frame extraction unit outputs the processed frame to the output unit.
The output unit takes out every 256 samples per one clock from a frame (512 samples outputted every two clocks) outputted from the IFFT output frame extraction unit and outputs them to the subsequent stage in the form of parallel signals.
The digital coherent receiver described in PTL 1 includes circuitry for performing the overlap-type FFT and IFFT that handle the above-described processes to prevent discontinuous points from occurring.